The present invention relates to a process for manufacturing pneumatic tires having a toric profile with a high transverse curvature, comprising a carcass, a tread band and a reinforcing annular structure, also known as belt, inserted between said carcass and said tread band.
Particularly, the invention concerns processes for preparing a tire carcass separately from the assembled tire, that assembly comprising a reinforcing annular structure and a tread band and then transferring said assembly onto the carcass for assembling and torically shaping the whole pneumatic tire.
As known, pneumatic tires having a high transverse curvature are those especially suitable for equipping two wheel vehicles such as motorcycles which, when cornering, operate with camber angles of 50.degree. or even 60.degree., values which are impossible to reach with conventional pneumatic tires for motor vehicles having a usual flat transverse profile.
In the manufacture of these pneumatic tires having a high transverse curvature a process is known comprising the steps of preparing the reinforcing annular structure separately from the carcass by radially superimposing layers of rubberized fabric reinforced with mutually crossed cords, in a known way, and of superimposing a tread band upon this reinforcing structure, before being assembled to the carcass.
In more detail, these pneumatic tires are usually manufactured by preparing the carcass on a suitable expansible building drum, in cylindrical configuration, and then bringing the configuration of the carcass to a first toric shape. Then two coaxial hollow supports are moved near the carcass sidewalls, said supports having a radially outer cylindrical surface (the well known bells) and a diameter like that of the carcass in the above said first toric shape. By means of said supports it is possible to arrange on said carcass, at its center line left free by the bells, in flat configuration and for subsequent steps, the reinforcing layers and the tread band, making them adhere the one to the other and to the carcass center line portion by stitching of the central portion of the assembly constituted by the reinforcing layers and the tread band.
At this stage the bells are moved away and the lateral zones of the assembly are stitched so as to make the remaining portions (lateral) of the reinforcing layers and tread band adhere to the carcass, which meanwhile is shaped according to a second toric configuration.
The so obtained green tire is therefore inserted into the Vulcanizing mould where it undergoes a further shaping consisting of an increase of the circumferential development of the tire of about 5%, during which the tread pattern is molded into the tread band.
A second known process consists of preparing the carcass in the form of a cylindrical sleeve on a main expansible drum, preparing on a separate ancillary drum the reinforcing annular structure, expanding said ancillary drum so far as to bring said reinforcing annular structure to a first toric configuration, applying the tread band on the so shaped annular structure stitching it with suitable means in order to provide it with the same toric configuration as the above said annular structure, transferring the assembly constituted by structure and tread band in correspondence of the carcass mid-plane, coaxially to the same, and expanding this carcass to the same toric configuration as the assembly in order to effect the assembling of the two elements and consequently the green tire to be vulcanized. This process and the relative apparatus are described in the Italian patent No. 1,099,513 (U.S. Pat. No. 4,288,265) of the present assignee.
Also in this case vulcanization of the pneumatic tire takes place in the conventional way, after having brought the above said pneumatic tire to its final toric shaping inside the mold.
The above processes cause different difficulties and problems owing to the stitching operation exerted on the tread band and to the final configuration carried out inside the mold, which result in a product that does not correspond to the qualitative characteristics required; in fact, the stitching is a very strong operation that causes deformations in the tread band with transfer of elastomeric material from the center towards the ends of the band (in this operative step the band is still in the green state, i.e. plastic state) as well as uncontrollable and undesired angular variations of the reinforcing cords of the belt layers and also of those of the carcass plies, both when the stitching is carried out after the assembling of the annular element (belt/tread assembly) to the carcass, and during the shaping inside the mould. All these drawbacks produce a decrease in quality of the resulting pneumatic tire. Today such variations no longer are tolerable in view of the high performance required not only in racing use, but also in less demanding uses such as normal tourist use.